In recent years, with the spread of portable electronic equipment such as portable telephones and notebook-sized personal computers, there is a strong need for development of a compact and lightweight nonaqueous-electrolyte secondary battery having high energy density. There is also a strong need for development of a high-output secondary battery as the power source for driving a motor, and particularly as the battery of the power source of transport equipment.
As a secondary battery that satisfies such a demand, there is a lithium ion secondary battery. A lithium ion secondary battery includes an anode, a cathode, an electrolyte and the like, and as the active material for the anode and cathode, a material capable of insertion and desorption of lithium is used.
Currently, much research and development of various lithium ion secondary batteries is being carried out, and among them, lithium ion secondary batteries that use a lithium metal composite oxide with layered structure or spinel structure for the cathode material are capable of obtaining a 4V-class high voltage, so practical application of these batteries having high energy density is advancing.
Currently, as the cathode material of this kind of lithium ion secondary battery, lithium composite oxides such as lithium cobalt composite oxide (LiCoO2) for which synthesis is relatively easy, lithium nickel composite oxide (LiNiO2) that uses nickel that is less expensive than cobalt, lithium nickel cobalt manganese composite oxide (LiNi1/3Co1/3Mn1/3O2), lithium manganese composite oxide (LiMn2O4) that use manganese, and lithium nickel manganese composite oxide (LiNi0.5Mn0.5O2) have been proposed.
Of these cathode active materials, in recent years, much attention has been placed on lithium nickel composite oxide (LiNiO2), which has high capacity without using cobalt of which there are only small reserves, and furthermore, lithium nickel manganese composite oxide (LiNi0.5Mn0.5O2), which has excellent thermal stability. Lithium nickel manganese composite oxide (LiNi0.5Mn0.5O2) is a layered compound as in the case of lithium cobalt composite oxide and lithium nickel composite oxide, and the transition metal site basically includes nickel and manganese at a composition ratio of 1:1 (refer to Chemistry Letters, Vol. 30 (2001), No. 8, p. 744).
Incidentally, as a condition for obtaining a lithium ion secondary battery having good performance (high cycle characteristics, low resistance, high output), the cathode material should have particles having a uniform and suitable particle size.
This is because when a cathode material having an excessively large particle size and low specific surface area is used, it is not possible to sufficiently maintain the reaction area that reacts with the electrolyte, and thus the reaction resistance increases and it is not possible to obtain a battery with high output. Moreover, using a cathode material having a wide particle size distribution causes the voltage that is applied to the particles inside the electrode to not be uniform, and when discharging and charging is repeatedly performed, minute particles are selectively deteriorated, resulting in a decrease in capacity.
In aiming for an increase in the output of a lithium ion secondary battery, shortening the migration length of lithium ions between the cathode and anode is effective, so manufacturing a thin cathode plate is desirable, and from this aspect as well, it is useful to use cathode material having a desired particle size that does not include a large particle size.
Therefore, in order to improve the performance of the cathode material, it is important that the lithium nickel composite oxide, which is the cathode active material, be manufactured so as to have particles having a uniform and suitable particle size.
A lithium nickel composite oxide is normally manufactured from a composite hydroxide, so from the aspect of making the particle size of the particles of lithium nickel composite oxide uniform, it is necessary to use a composite hydroxide having uniform particle size as the raw material.
In other words, from the aspect of manufacturing a high-performance lithium ion secondary battery as a final product by improving the performance of the cathode material, it becomes necessary to use a composite hydroxide that is composed of particles having a narrow particle size distribution as the composite hydroxide that will become the raw material of the lithium nickel composite oxide of the cathode material.
As the nickel composite hydroxide that is used as the raw material for the lithium nickel composite oxide, there is, for example, a composite hydroxide, of which the ratio of manganese to nickel is essentially 1:1, disclosed in JP 2004-210560 (A) in which a manganese nickel composite hydroxide is characterized by an average particle size of 5 μm to 15 μm, a tap density of 0.6 g/ml to 1.4 g/ml, a bulk density of 0.4 g/ml to 1.0 g/ml, a specific surface area of 20 m2/g to 55 m2/g, a sulfate radical content of 0.25% to 0.45% by weight, and in X-ray diffraction, a ratio (I0/I1) of the maximum intensity (I0) of the peak at 15≤2θ≤25 and the maximum intensity (I1) of the peak at 30≤2θ≤40 that is 1 to 6. Moreover, the surface structure and internal structure of the secondary particles are such that the secondary particles are formed into a netlike shape that is a collection of pleated walls formed from primary particles, with the space surrounding the pleated walls being relatively large.
Furthermore, as a manufacturing method thereof, a method is disclosed in which the amount of manganese ion oxidation is controlled within a fixed range, and where in an aqueous solution having a pH value of 9 to 13 and with the existence of a complexing agent, a mixed aqueous solution of manganese salt and nickel salt having an atomic ratio of manganese and nickel that is essentially 1:1 is caused to react with an alkali solution under a suitable stirring condition, which causes co-precipitation of the resulting particles.
However, in the case of the lithium manganese nickel composite oxide and manufacturing method thereof that are disclosed in JP 2004-210560 (A), although the structure of the particles is investigated, as can be clearly seen from the disclosed electron micrograph, coarse particles and minute particles are mixed in the obtained particles, and therefore uniformity of the particle size has not been investigated.
On the other hand, in regards to the particle size distribution of lithium composite oxide, JP 2008-147068 (A), for example, discloses a lithium composite oxide in which the particles have an average particle size D50, which is the particle size of a cumulative frequency of 50% in the particle size distribution curve, of 3 μm to 15 μm, a minimum particle size of 0.5 μm or greater and a maximum particle size of 50 μm or less, and where in the relationship among average particle size D50, average particle size D10 at a cumulative frequency of 10% and D90 at a cumulative frequency of 90%, D10/D50 is 0.60 to 0.90, and D10/D90 is 0.30 to 0.70. It is also disclosed that this lithium composite oxide has high repletion, excellent charge and discharge capacity characteristics, and does not readily degrade even under conditions of a large charge and discharge load, so by using this lithium composite oxide, it is possible to obtain a lithium ion nonaqueous-electrolyte secondary battery with less degradation in cycle characteristics.
However, even though the lithium composite oxide that is disclosed in JP 2008-147068 (A) has an average particle size of 3 μm to 15 μm, the minimum particle size is 0.5 μm or greater and the maximum particle size is 50 μm or less, so minute particles and coarse particles are included. Moreover, the particle distribution is regulated by D10/D50 and D10/D90 above, so it cannot be said that the particle size distribution is narrow. In other words, the lithium composite oxide disclosed in JP 2008-147068 (A) cannot be said to have particles that have sufficiently high particle uniformity, and by using this lithium composite oxide, an improvement in performance of the cathode material cannot be expected, and it is difficult to obtain a lithium ion nonaqueous-electrolyte secondary battery having sufficient performance.
Furthermore, a manufacturing method for the composite hydroxide that will become the raw material composite oxide has also been proposed with the objective of improving the particle size distribution. In JP 2003-86182, a manufacturing method for the cathode active material for a nonaqueous-electrolyte battery is proposed in which an aqueous solution that includes two or more kinds of transition metal salts, or an aqueous solution and alkali solution of two or more kinds of different transition metal salts are simultaneously put into a reaction vessel, and by causing co-precipitation while coexisting together with a reducing agent, or while passing an inert gas through the solution, a hydroxide or oxide is obtained as a precursor.
However, the technology of JP 2003-86182 classifies and collects the generated crystals, so in order to obtain a product having a uniform particle size, strict management of the manufacturing conditions is considered to be necessary, and thus production on an industrial scale is difficult. Moreover, uniformity of particle size is achieved through classification, so the degree of uniformity will not exceed the classification precision.
Furthermore, in order to increase the output of a battery, increasing the reaction surface area without changing the particle size is effective. In other words, by making the particles porous or making the particle structure hollow, it is possible to increase the surface area that contributes to the battery reaction, and thus it becomes possible to reduce the reaction resistance.
For example, in JP 2004-253174, a cathode active material for a nonaqueous-electrolyte secondary battery that has at least a layered lithium transition metal composite oxide, where the lithium transition metal composite oxide is composed of hollow particles having an outer-shell section on the outside and a space on the inside of the outer-shell section. It is also disclosed that this cathode active material for a nonaqueous-electrolyte secondary battery has excellent battery characteristics such as cycle characteristics, output characteristics, thermal stability characteristics and the like, and can be suitably used in a lithium ion secondary battery.
The cathode active material that is disclosed in JP 2004-253174 (A) has hollow particles, so the increase in specific surface area is expected compared to solid particles, however, it makes no mention of particle size thereof. Therefore, an improvement in reactivity with the electrolyte due to the increase in specific surface area can be expected, however, the effect on the migration length of lithium ions due to making the particles minute is unclear, and sufficient improvement of the output characteristic cannot be expected. Furthermore, in regards to the particle size distribution, the distribution is considered to be the same as that of conventional cathode active material, so there is a high probability that selective degradation of minute particles due to lack of uniformity of the applied voltage inside the electrodes will occur, and that the battery capacity will drop.
As described above, at the current time neither a lithium composite oxide that sufficiently improves the performance of a lithium ion secondary battery nor a composite oxide that is the raw material for that composite oxide have been developed. Moreover, various methods for manufacturing a composite hydroxide have been investigated, however, on an industrial scale, a method capable of manufacturing a composite hydroxide that will become the raw material of a composite oxide that is able to sufficiently improve the performance of a lithium ion secondary battery has not been developed. In other words, development of a cathode active material having a suitable particle size, and particularly, having good particle size uniformity and suitable a particle size of about 8 μm to 16 μm, and furthermore, a cathode active material having a large reaction surface area, for example, having hollow structure, has not been performed, and there is a need for development of such a cathode active material and industrial manufacturing method thereof.